- John studied with William Esper and Deborah Van Valkenburgh.
- He is Irish-American.
- John shares his birthday with many celebrities such as Sarah Michelle Gellar and Adrien Brody.
- John's son, Jake attends the University of Miami studying business.
- Two of John's siblings are chiropractors.
- He is a former assistant director at Chelsea Westside Theater and a former teacher at Pratt Institute.
- John has an Abyssinian cat named Pharaoh.
- He enjoys practicing yoga and tai chi on a regular basis. When he's not busy, he spends time with his family.
- He was a football and track athlete in school.
- On stage he received a Drama Desk award for American Days, an Obie award for The Dining Room, and a Theatre World Award Yentl.
- He was first discovered after his portrayal of Joseph in The Nativity (1978).
- He is a regular reader on Selected Shorts which is broadcast nationwide on Public Radio International.
- John is a founding member of the Nantucket Film Festival.
- John has two sons named Jake and Caiden. He also has a daughter, Miranda.
- In 1988, John won an Emmy for Outstanding Supporting Actor in a Miniseries or a Special for Baby M. In 1998, John won an American Independent Award for Southie.
- He was married to Laura Pettibone for 30 years.
- As of 2001, John is married to Melissa MacLeod.
- He is 6'0½" tall.
- (On Adam Kane, the leader in Mutant X)
John: It's fun for me because I am a father in real life, so I understand that parental thing.
- (On his character, Adam Kane in Mutant X)
John: I think of him more as a kind of leader or a coach or a big brother. Rather than think of myself as their father, I think more of myself as an older brother who is maybe a little bit wiser about the ways of the world. Who can inspire them and lead them. A leader, if you will. A director who is directing them into a path.
- (On his role of Lex Luthor)
John: I did every single episode the first season. I was living in New York when I shot Lois and Clark and I was commuting to Los Angeles every week to shoot. It was very difficult on me and my family. So I asked them if I could get out of my contract at the end of the first season, and they wouldn't let me out altogether, but they let me diminish my participation so that I only did certain episodes that were called Lex Luthor episodes.
- (On his character, Adam Kane in Mutant X)
John: I think Adam’s story is one of betrayal, you know? He’s somebody who believed he’s an idealist. He’s a wounded idealist. I think he’s felt the sharp pain of a knife squarely placed between his shoulder blades. I think a lot of what he does is trying to recover from this betrayal.
- (On his portrayal of Lex Luthor)
John: I don't think there has been a character quite like this. This villain is written as a human being, and not just a one-dimensional comic villain or a two-dimensional comic villain, the way he might have been portrayed in the comic books or even other films. I thought Gene Hackman was brilliant, but it was a different kind of comic villain.
- John: When I read a role, I try to find something that I can bring to the role and something that the role brings to me. Something that I don't know anything about and that the character can teach me.
