diff --git a/entry.go b/entry.go
index 9f0a5e1..ba705b5 100644
--- a/entry.go
+++ b/entry.go
@@ -7,9 +7,6 @@ import (
 	"time"
 )
 
-// assert interface compliance.
-var _ Interface = (*Entry)(nil)
-
 // Now returns the current time.
 var Now = time.Now
 
@@ -32,7 +29,14 @@ func NewEntry(log *Logger) *Entry {
 }
 
 // WithFields returns a new entry with `fields` set.
-func (e *Entry) WithFields(fields Fielder) *Entry {
+func (e *Entry) WithFields(fields Fielder) Interface {
+	return e.withFields(fields)
+}
+
+// withFields returns a new entry with `fields` set.
+// but to satify the interface WithFields casts *Entry
+// back into `Interface`
+func (e *Entry) withFields(fields Fielder) *Entry {
 	f := []Fields{}
 	f = append(f, e.fields...)
 	f = append(f, fields.Fields())
@@ -43,7 +47,7 @@ func (e *Entry) WithFields(fields Fielder) *Entry {
 }
 
 // WithField returns a new entry with the `key` and `value` set.
-func (e *Entry) WithField(key string, value interface{}) *Entry {
+func (e *Entry) WithField(key string, value interface{}) Interface {
 	return e.WithFields(Fields{key: value})
 }
 
@@ -51,7 +55,7 @@ func (e *Entry) WithField(key string, value interface{}) *Entry {
 //
 // The given error may implement .Fielder, if it does the method
 // will add all its `.Fields()` into the returned entry.
-func (e *Entry) WithError(err error) *Entry {
+func (e *Entry) WithError(err error) Interface {
 	ctx := e.WithField("error", err.Error())
 
 	if s, ok := err.(stackTracer); ok {
@@ -129,9 +133,9 @@ func (e *Entry) Fatalf(msg string, v ...interface{}) {
 
 // Trace returns a new entry with a Stop method to fire off
 // a corresponding completion log, useful with defer.
-func (e *Entry) Trace(msg string) *Entry {
+func (e *Entry) Trace(msg string) Interface {
 	e.Info(msg)
-	v := e.WithFields(e.Fields)
+	v := e.withFields(e.Fields)
 	v.Message = msg
 	v.start = time.Now()
 	return v
diff --git a/entry_test.go b/entry_test.go
index 328106b..a850a38 100644
--- a/entry_test.go
+++ b/entry_test.go
@@ -13,9 +13,9 @@ func TestEntry_WithFields(t *testing.T) {
 
 	b := a.WithFields(Fields{"foo": "bar"})
 	assert.Equal(t, Fields{}, a.mergedFields())
-	assert.Equal(t, Fields{"foo": "bar"}, b.mergedFields())
+	assert.Equal(t, Fields{"foo": "bar"}, b.(*Entry).mergedFields())
 
-	c := a.WithFields(Fields{"foo": "hello", "bar": "world"})
+	c := a.WithFields(Fields{"foo": "hello", "bar": "world"}).(*Entry)
 
 	e := c.finalize(InfoLevel, "upload")
 	assert.Equal(t, e.Message, "upload")
@@ -28,19 +28,19 @@ func TestEntry_WithField(t *testing.T) {
 	a := NewEntry(nil)
 	b := a.WithField("foo", "bar")
 	assert.Equal(t, Fields{}, a.mergedFields())
-	assert.Equal(t, Fields{"foo": "bar"}, b.mergedFields())
+	assert.Equal(t, Fields{"foo": "bar"}, b.(*Entry).mergedFields())
 }
 
 func TestEntry_WithError(t *testing.T) {
 	a := NewEntry(nil)
 	b := a.WithError(fmt.Errorf("boom"))
 	assert.Equal(t, Fields{}, a.mergedFields())
-	assert.Equal(t, Fields{"error": "boom"}, b.mergedFields())
+	assert.Equal(t, Fields{"error": "boom"}, b.(*Entry).mergedFields())
 }
 
 func TestEntry_WithErrorFields(t *testing.T) {
 	a := NewEntry(nil)
-	b := a.WithError(errFields("boom"))
+	b := a.WithError(errFields("boom")).(*Entry)
 	assert.Equal(t, Fields{}, a.mergedFields())
 	assert.Equal(t, Fields{
 		"error":  "boom",
diff --git a/interface.go b/interface.go
index c92ebea..689dd47 100644
--- a/interface.go
+++ b/interface.go
@@ -1,10 +1,18 @@
 package log
 
+// Fields represents a map of entry level data used for structured logging.
+type Fields map[string]interface{}
+
+// Fielder is an interface for providing fields to custom types.
+type Fielder interface {
+	Fields() Fields
+}
+
 // Interface represents the API of both Logger and Entry.
 type Interface interface {
-	WithFields(fields Fielder) *Entry
-	WithField(key string, value interface{}) *Entry
-	WithError(err error) *Entry
+	WithFields(fields Fielder) Interface
+	WithField(key string, value interface{}) Interface
+	WithError(err error) Interface
 	Debug(msg string)
 	Info(msg string)
 	Warn(msg string)
@@ -15,5 +23,5 @@ type Interface interface {
 	Warnf(msg string, v ...interface{})
 	Errorf(msg string, v ...interface{})
 	Fatalf(msg string, v ...interface{})
-	Trace(msg string) *Entry
+	Trace(msg string) Interface
 }
diff --git a/logger.go b/logger.go
index 1755747..c0cf7b3 100644
--- a/logger.go
+++ b/logger.go
@@ -5,16 +5,7 @@ import (
 	"sort"
 )
 
-// assert interface compliance.
-var _ Interface = (*Logger)(nil)
-
-// Fielder is an interface for providing fields to custom types.
-type Fielder interface {
-	Fields() Fields
-}
-
-// Fields represents a map of entry level data used for structured logging.
-type Fields map[string]interface{}
+// note: Fields is defined in `interface.go`
 
 // Fields implements Fielder.
 func (f Fields) Fields() Fields {
@@ -62,7 +53,7 @@ type Logger struct {
 }
 
 // WithFields returns a new entry with `fields` set.
-func (l *Logger) WithFields(fields Fielder) *Entry {
+func (l *Logger) WithFields(fields Fielder) Interface {
 	return NewEntry(l).WithFields(fields.Fields())
 }
 
@@ -70,12 +61,12 @@ func (l *Logger) WithFields(fields Fielder) *Entry {
 //
 // Note that the `key` should not have spaces in it - use camel
 // case or underscores
-func (l *Logger) WithField(key string, value interface{}) *Entry {
+func (l *Logger) WithField(key string, value interface{}) Interface {
 	return NewEntry(l).WithField(key, value)
 }
 
 // WithError returns a new entry with the "error" set to `err`.
-func (l *Logger) WithError(err error) *Entry {
+func (l *Logger) WithError(err error) Interface {
 	return NewEntry(l).WithError(err)
 }
 
@@ -131,7 +122,7 @@ func (l *Logger) Fatalf(msg string, v ...interface{}) {
 
 // Trace returns a new entry with a Stop method to fire off
 // a corresponding completion log, useful with defer.
-func (l *Logger) Trace(msg string) *Entry {
+func (l *Logger) Trace(msg string) Interface {
 	return NewEntry(l).Trace(msg)
 }
 
diff --git a/logger_test.go b/logger_test.go
index cd04c44..7c4813e 100644
--- a/logger_test.go
+++ b/logger_test.go
@@ -93,7 +93,7 @@ func TestLogger_Trace_info(t *testing.T) {
 	}
 
 	func() (err error) {
-		defer l.WithField("file", "sloth.png").Trace("upload").Stop(&err)
+		defer l.WithField("file", "sloth.png").Trace("upload").(*log.Entry).Stop(&err)
 		return nil
 	}()
 
@@ -124,7 +124,7 @@ func TestLogger_Trace_error(t *testing.T) {
 	}
 
 	func() (err error) {
-		defer l.WithField("file", "sloth.png").Trace("upload").Stop(&err)
+		defer l.WithField("file", "sloth.png").Trace("upload").(*log.Entry).Stop(&err)
 		return fmt.Errorf("boom")
 	}()
 
@@ -156,7 +156,7 @@ func TestLogger_Trace_nil(t *testing.T) {
 	}
 
 	func() {
-		defer l.WithField("file", "sloth.png").Trace("upload").Stop(nil)
+		defer l.WithField("file", "sloth.png").Trace("upload").(*log.Entry).Stop(nil)
 	}()
 
 	assert.Equal(t, 2, len(h.Entries))
@@ -236,13 +236,13 @@ func BenchmarkLogger_large(b *testing.B) {
 			"size": 1 << 20,
 		}).
 			WithFields(log.Fields{
-			"some":     "more",
-			"data":     "here",
-			"whatever": "blah blah",
-			"more":     "stuff",
-			"context":  "such useful",
-			"much":     "fun",
-		}).
+				"some":     "more",
+				"data":     "here",
+				"whatever": "blah blah",
+				"more":     "stuff",
+				"context":  "such useful",
+				"much":     "fun",
+			}).
 			WithError(err).Error("upload failed")
 	}
 }
diff --git a/pkg.go b/pkg.go
index 9bf51dc..1b7b813 100644
--- a/pkg.go
+++ b/pkg.go
@@ -29,17 +29,17 @@ func SetLevelFromString(s string) {
 }
 
 // WithFields returns a new entry with `fields` set.
-func WithFields(fields Fielder) *Entry {
+func WithFields(fields Fielder) Interface {
 	return Log.WithFields(fields)
 }
 
 // WithField returns a new entry with the `key` and `value` set.
-func WithField(key string, value interface{}) *Entry {
+func WithField(key string, value interface{}) Interface {
 	return Log.WithField(key, value)
 }
 
 // WithError returns a new entry with the "error" set to `err`.
-func WithError(err error) *Entry {
+func WithError(err error) Interface {
 	return Log.WithError(err)
 }
 
@@ -95,6 +95,6 @@ func Fatalf(msg string, v ...interface{}) {
 
 // Trace returns a new entry with a Stop method to fire off
 // a corresponding completion log, useful with defer.
-func Trace(msg string) *Entry {
+func Trace(msg string) Interface {
 	return Log.Trace(msg)
 }
diff --git a/pkg_test.go b/pkg_test.go
index f233cf7..e980dd2 100644
--- a/pkg_test.go
+++ b/pkg_test.go
@@ -71,6 +71,6 @@ func Example_multipleFields() {
 // Trace can be used to simplify logging of start and completion events,
 // for example an upload which may fail.
 func Example_trace() (err error) {
-	defer log.Trace("upload").Stop(&err)
+	defer log.Trace("upload").(*log.Entry).Stop(&err)
 	return nil
 }