Who played Sam Beckett in 'Quantum Leap'?

Who played Screech Powers in 'Saved by the Bell'? In which TV series is Screech Powers a main character? Who played Steve Urkel in 'Family Matters'? In which TV series is Steve Urkel a main character? In which TV series is Mitch Buchannon a main character? Who played C.J. Parker in 'Baywatch'? In which TV series is C.J. Parker a main character? Who played Dale Cooper in 'Twin Peaks'?