He first appeared on Doctor Who as the caretaker (voice) on Scream of the Shalka, however, his first role as The Doctor was on 18th June 2005, as he regenerated from Christopher Eccleston's Doctor.
Also played Barty Crouch Jr in Harry Potter and the Goblet of Fire.
Girl #2: OH MY GOD! It's totally David Tennant!
Really? I wish I could get David Tennant!