Again, the keepers aren't that smart. They're a choke chain meant to correct wrong behaviour. They *cannot* take total control of a person's mind, nor dictate their every word and they're only as cunning or attentive as the Drakh at the other end of the link.
Is that speculation on your part (in which case it’s no more valid or invalid than what Kirk55555 suggested) or something which is mentioned in the books.
Either way though, sticking with what is seen and said on screen. Captain Jack (Racing Mars) had a keeper, and while he could drop some subtle hints that no one picked up on at the time, it was essentially controlling what he said and did. Swapping ident cards, lying, trying to shoot the leader of the Mars resistance. Dialog from that episode also points out the level of control they have - and it goes a bit beyond just correcting wrong behaviour.
Captain Jack (after it was partially removed) - “I wanted to warn you, but I couldn’t say anything. It wouldn’t let me”
Dr Franklin - “This entity can cut into the neuro pathways and override them. Some of the micro fibres are fresh, so my guess is it’s control over a given host grows as it does.”
Londo would have had one on him for a couple of decades, or there abouts.