As a director, I would try to bring out the comedy in this extract by, firstly, casting two people who have an ability to make people laugh by doing hardly anything. There are people who can send an audience into fits of laughter by simply pulling a face or walking. A naturally funny person would make the comic extracts of this play easier for the audience to accept.
As for the direction of these actors, I would ask them to perform certain lines as follows: When Bri first mentions Freddie, his obvious dislike of the man could be very funny. I would ask the actor playing Bri to say Freddie's name with absolute disgust. Later, when Sheila reminds Bri that it was he who first introduced her to Freddie, Bri's reaction (a wry smile perhaps) would continue this joke. When Sheila tries to persuade Bri to go to the rehearsal by offering him whisky afterwards, Bri's reaction of 'whisky first, Freddie after' is easy for the audience to empathise with and thus laugh at.
I would ask the actor playing Bri to say this line as if it were preposterous that the whisky were to come after the gruelling prospect of a conversation with Freddie.
Sheila's annoyance with the stupidity of the cat is very funny. If the actress were to have quite a lot of difficulty getting the door open because of the cat and then kicking it (with a pre-recorded squeal audible over loud-speakers), her mini-duel would be quite entertaining. Similarly amusing is the stupidity of talking to the cat and the audience's realisation of how stupid it is, yet they all do it.
When Bri and Sheila are speaking to Joe (or rather, speaking at her), their mock enthusiasm and amazement is very funny. To emphasize this...