1966-1969. Honorable mentions: Red Buttons, Leonard Stone, Peter Graves and Martin Landau
Leslie Nielsen wrapped up two films in 1966 (The Plainsman, Beau Geste) just before auditioning for Doctor Who. Being cast as a tough guy for most of his Hollywood career, he relished the chance to utilize his comedy skills as the second Doctor.
Stefanie Powers thankfully chose Doctor Who over a role in The Girl from U.N.C.L.E., which turned out to be a 1-season flop. They wanted a “demure, passive figure” instead of an action heroine like Emma Peel. Luckily for generations of Who fans, Powers found the role of Polly Wright a lot more interesting.
Stephen Brooks earned respect as a character actor in Route 66, The Doctors and the Nurses, 12 O’Clock High and The F.B.I. (a show he abruptly left after auditioning for Doctor Who). His audition went so well, he unknowingly beat out a young Michael Craze).