Whitmore embodied the role of Brooks Hatlen in Frank Darabont's The Shawshank Redemption (1994) with rueful grace and heartfelt regret. As the longtime librarian, he was a wise and kind friend to the initially floundering Andy (Tim Robbins). But he had absolutely no idea how to live outside the prison walls. This scene (link to a big spoiler if you haven't seen the movie) with just the right touch in the narration by Whitmore as he writes to his buddies, is heartbreaking, and I found it impossible to watch all the way through after hearing the news of his passing.
My personal, sentimental favorite remains Them! (1954), the greatest giant ant movie ever made. Whitmore played a New Mexican police Sergeant who finds a little girl wandering in the desert, and soon finds himself courageously facing down nature gone amuck. He was so friendly and calm, yet authoritative, that I never doubted for a moment that giant ants were a serious threat to mankind.
He was much more fiery in Give 'em Hell, Harry! (1975), for which he was nominated for an Academy Award. The film was an adaptation of a one-man stage play about President Harry S Truman. I was spellbound in the movie theater for 100 minutes, held captive once more by Whitmore's authority, but also by the sparkle in his eyes as his moved brusequely around the stage.