Richards interrogated Nina Myers; however, she attempted to commit suicide shortly afterwards on one of his needles.
Richards was about to interrogate Richard Heller, but Curtis Manning was concerned about the morality of the decision, and so he stopped him before he could inflict any pain.
Later in the day, Richards interrogated Sarah Gavin because she was suspected of being a mole. Once Marianne Taylor was taken into custody, he was stopped.
Curtis Manning called in Richards to interrogate Behrooz Araz; Richards may have administered truth serum, as Behrooz seemed sleepy and compliant after the injection.
He was about to interrogate Joe Prado, but a lawyer named David Weiss arrived and forced CTU to release the suspect.
Background information and notes
Richards is one of three CTU LA agents whose sole known function is interrogation. Agent Johnson was an interrogator active during Season 3 and mentioned during Season 4, and Rick Burke took on this role during Seasons 5 and 6.
In his Season 3 appearance, his first name is given as Darren, but there is no other indication that "Eric Richards" was meant to be a different character in the following season. Whether one of the names might be his nickname or middle-name is unknown.