I'm slowly making my way through I, Claudius on Acorn, and saw this episode last night. That's a 36-year-old Patrick Stewart as Sejanus copping a feel off of Patricia Quinn's Livilla, a daughter-in-law of Tiberius. It's from the POV of Livilla's dying husband (whom she poisoned, because I, Claudius). This was a year after RHPS for Quinn, but a decade before Stewart commanded the Enterprise, and even a few years before he was in Excalibur, which is the first thing I saw him in.
Earlier, they have a love scene in which Livilla bares a breast and Sejanus bares a hairy back, and it's just super creepy (and not just because the characters are super creepy, which they are).
It's yet another quirk of British actors -- so few of them that they keep turning up in the same places, often in unexpected (like this) places. There's a lot of overlap between I, Claudius and Doctor Who, for example (Quinn turned up in Seven's "Dragonfire," though somehow Stewart missed out on DW). ETA: So I actually got curious and took a look at the overlap, and in addition to Quinn, there's Brian Blessed, George Baker, Fiona Walker, Kevin McNally and of course John Hurt and Derek Jacobi. And that's just of people who were in four or more I, Claudius episodes; the cast list was so big, that I didn't check three or under (though I did see Sheila Ruskin in that list). How did Stewart avoid being on Classic Who?