Peter Cushing: A One-Way Ticket to Hollywood (1989)
https://archive.org/details/one-way-to-hollywood-1989-feature-length-interview-with-peter-cushing - uploaded to archive.org by stewarwd85 (49 minutes long, skip to 58:29 for the Star Wars segment.)

The blurb
“Legendary British actor Peter Cushing (1913-1994) talks about his career in TV and film and his early years in Hollywood, while sharing private memories and anecdotes in his usual delightfully humorous way.”

Just the Star Wars segment from this documentary:

Peter Cushing on playing Grand Moff Tarkin in carpet slippers
https://www.youtube.com/watch?v=NdXLDvx_tHQ - from the Eyes On Cinema YouTube channel (2 mins long).

Backup link from the Grand Moff Tarkin YouTube channel:
https://www.youtube.com/watch?v=jpdaVCBcaIk