On the Down Low. Nobody is Supposed to Know
If anyone wants to make an All the President's Men computer game, in the style of Telltale's popular Walking Dead game the writing would be easy. The story is already filled with dozens of secondary characters called CREEPs, which is a great name for a zombie-type character. (If they bite you, you transform into a Richard Nixon lookalike.)
Proving that truth is stranger than fiction, the corrupt Committee to Re-Elect the President is given the appropriate acronym of CREEP. If this were a novel, the author would be accused of being cheesy and unimaginative. But this is real life, not fantasy.
The vast majority of characters in All the President's Men exist because they existed in real life. They're sources of information for our main characters, and nothing more. Here are two the real-life figures who show up to spout some info in All the President's Men.
Hugh Sloan is treasurer of the Committee to Re-Elect the President. He wants to tell the truth, but fears the repercussions of it. He's honest with the information he does tell the reporters, although that isn't much.
Second is Donald Segretti, who seems to be a professional troublemaker. His job is to literally manufacture scandals for the democrats. (We bet he wishes he could have done his job in the Twitterverse, and not in the analog 70's.)
One person we never see in All the President's Men is the POTUS: Richard Nixon. Nixon is like the Godot we're all waiting for, and he never shows up.