Which actor portrayed the informant Mr. X?

Steven Williams portrayed the informant known as Mr. X on The X-Files.

Mr. X is a shadowy source who leaks information about paranormal cases to FBI agents Fox Mulder and Dana Scully. He was introduced as a replacement for Deep Throat, Mulder's first major informant, after Deep Throat was killed in the first-season finale, "The Erlenmeyer Flask." Unlike his predecessor, Mr. X is reluctant, intimidating and often appears to help only when doing so serves his own interests.

Williams brought a restrained but forceful presence to the role. The character's identity is generally referred to simply as X, although Mulder and others also call him Mr. X. He is connected to the Men in Black and has access to highly sensitive information, but his loyalties remain uncertain. That ambiguity is part of why he became one of Mulder's most memorable informants.

Mr. X is killed in the second-season episode "Herrenvolk" after trying to pass information to Mulder. His death leads to the introduction of Marita Covarrubias as another source. Williams later returned as X in the series finale, "The Truth," and in the 2002 episode "This."