This fall, Freddie Prinze Jr. will do two episodes of David E. Kelley's Practice spin-off, Boston Legal, Variety reports. He'll play the underachieving son of William Shatner's Denny Crane. ER's Elizabeth Mitchell will also guest-star as the loony ex-lover of James Spader's Alan Shore. Meanwhile, Aidan Quinn will appear in several episodes of NBC's Third Watch as a police lieutenant.