Complex sentence: She began her career as a child actor, performing in sketches on "The Tonight Show with Jay Leno" before appearing in supporting roles, most notably in "Sleepover" and "13 Going on 30", both in 2004.
Split into simpler sentences: She began her career as a child actor, performing in sketches on "The Tonight Show with Jay Leno". She appeared in supporting roles, most notably in "Sleepover" and "13 Going on 30", both in 2004.