He gave him a list of 'no-go areas' that might arise, like Afghanistan. He said that concerning the Geneva talks, Manhattan should reply that they can't resume until the Soviets agree to exclude him from the agenda.
However, it was not what Manhattan was there for. After Doug Roth's allegations that Manhattan causes cancer to humans, a fray erupted and the journalists came towards him asking for details concerning his relationship to Janey Slater. Forbes attempted to guide Manhattan outside and hold off the journalists. Eventually, Manhattan teleported everyone away.
After Dr. Manhattan teleported to Mars, Forbes was at Rockefeller Military Research Center while army agents sealed their quarters. He ordered a cancer scan on Laurie Juspeczyk and asked her about whether he had put him in emotional stress, making her angry for considering herself responsible. Forbes replied equally angrily saying that the psychologists estimate that he won't return, and his absence will put everyone in trouble.
In the film, he is portrayed by Fulvio Cecere. In the director's cut DVD his role is somewhat expanded, as his interrogations make Laurie angry. She later beats and ties him with handcuffs in order to escape.