I always like William Devane, but you make an interesting point. I heard someone make a comment on Jack Nicholson's acting, and I do believe he is a great actor, that there were later movies in which he was just "being Jack Nicholson."
But in some cases like these, it's a chicken/egg situation because the casting agents and producers go into the process saying they want a certain actor (or a certain "type") because of their image. Maybe Jack Nicholson was there "playing Jack Nicholson" because that was what the producer wanted. Such typecasting can give a project built-in audience---people will show up at the theater just because they love Jack doing his thing. They might also show up if he was playing against type, but there would be a certain number of disappointed fans. If he gets paid regardless, why would he care? He's at a stage in his career when typecasting doesn't hurt his ability to get work.
In Devane's case, he does tend to take roles where he plays powerful, intelligent authority figures. Fans tend to expect that kind of performance from him, and he's always done well with them. If he were to take (for instance) a guest role in a sitcom where he played a dimwit, or a victim of elder abuse in a police procedural, it would be a departure from what we are accustomed to. It would still be very watchable, of course, but people would be surprised because a lot of producers don't even think to audition certain actors for roles that play against their usual work. I recall being rather amused when Donna Mills (glamorous, photogenic Donna Mills) took on the lead role in a stage production of
Driving Miss Daisy wearing the white wig, wire-rim glasses and high-collared dress. She did a great job, the critics agreed, but all the buzz was about how "sex kitten" Donna Mills was playing a role made famous by
Jessica Tandy.