I think that I liked him most as Sejanus in I, Claudius. That was before any of his major sci-fi roles.

I see him in this episode:

https://m.youtube.com/watch?v=xA6QbwPGlgk

I've read both Suetonius and Tacitus on the subject of Augustus and the Julio-Claudian dynasty:

https://en.wikipedia.org/wiki/Julio-Claudian_dynasty

Those who lack this family knowledge would want to start with the first episode of I, Claudius.