Terrible situation

Sorry to hear about your trouble. Terrible situation indeed.

The following approach worked okay for my first 15-minute short film with four actors to prepare them. The rationale behind this approach is that I find the actors the key element. Much more important than the time spent on technical or production related aspects. You can of course debate about that.

My budget was $3000 and the cast&crew was mostly amateurs (cast&crew working without salary). To me it is surprising that you seem to have more professional and bigger feature production so I am really surprised that you could not convince your producer to spend resources on rehearsing the actors.

Of course you can never be 100% sure about actors, but by making sure that your actors are committed to the production and spend effectively the time they have available for preparing, I think you will maximize their performance on the set.

My approach for effective working with actors in indie / low-budget film:
1) Find potential actors (distribute marketing material, talk to indie communities, networks, anywhere...).
2) Ask any interested actor to send CV and photo to you by email
3) Phone interview each actor that you think is potential a role
4) Preliminarily select the actors for each role
5) Invite the actors to a 3-hour kick-off / screen test session (introduce production, discuss the script, read-through, shoot some blocked read-throughs of the key scenes)
6) Give detailed feedback to actors about the screen test (within a week after the screentest)
7) Later, meet each actor individually and discuss about their character (1-2 hrs)
8) 1-2 weeks before the shooting starts organize a one-day rehearsal session for fine-tuning, blocking and improvisation (here you can check that they have memorized their lines and fine-tune scenes).
9) Have a production kick-off party (the actors to meet the crew in a relaxed environment before the set)
10) 1-2 days before the shoot, check (by phone) that everything is okay with the actors and prepare to spend some time discussing their concerns
11) On the set make sure that: a) if your budget is above zero, spend the first dollars on a make-up artist, b) there is an assistant who actors can easily turn to with any practical concern, c) keep the schedule (what will happen and when 100% clear to all actors all the time) d) atmosphere is calm and nobody distracts the actors, e) craft services available 100% of time.
12) Direct the actors, especially: a) give clear feedback of actors' performance, b) don't focus too much on the technical side

I also could have done a real audition, but having no prior practical experience of film production, I decided to do a light approach. This light approach was also good in the way that I was able to get one professional actor who is also acting in Finnish TV. If I would have auditioned him against 20 amateurs, I would have lost him and also wasted a hell of a lot of time from myself and everybody else.

Hope this gives some ideas and helps even a bit (at least for the next time) and good luck!

Yeah if he really can't carry the performance like you thought, you don't want to carry through with all the hard work only to have a -lead- that isn't convincing.

Have you told the casting director or producer? They of course need to know about this and will help you take the right steps in getting a new actor more fit for the role.