Trust your gut instinct: When my producer pointed out Andrew for our other lead character, the minute I looked at him, I just knew, actor or not that he would work. The rest of that process would be whether I have enough skill to bring out his acting, not whether he can act or not. During the screen test, others realised that he worked as well but even though it was suggested we look at other possibilities, I just knew he was the one. Trust your gut. It's usually right.
Use embarrassment: Romantic scenes are really hard to rehearse, especially with non-actors. I get embarrassed so I know they do. However, remember this - if your actor get embarrassed during rehearsal, and still carries on - get excited. It means there is a special spark there. They are willing to try and it also means something is happening internally that is real and has emotion, which technically as a director you can convert into energy for the performance. The real performers to fear are the ones who show barely any emotion, off screen or on. Generally it means you have nothing to work with.
The enemy of art is assumption
Agents are quite often unhelpful and sometimes act stupidly: Like everything, this has a few exceptions (i.e Kreative Talent Agency who have helped me out a lot and actually seem to care). I sent out countless emails to agents of named and unnamed actors looking for an actor for one of my roles. At first I thought it was wonderful. "I'm sorry, David Tennant won't be available at the time of your shooting." How marvelous. if he had been available they might have read the script! By the 5th reply that was almost word for word from every agent I contacted I decided that this is a sentence that the Evil Agents Guild came up with at their annual meeting back in the 1800s and thought was polite enough yet firm enough to get rid of any short filmmakers who won't make them any money. That's the unhelpful. As for the stupid - I sent out a casting call asking specifically for a male actor and received a reply telling me that "Mary" would be a great option for me to contact...
Pay your principal actors: Not just because it shows value to their work but also it's good for you. It gives you a reputation (hopefully) that you're serious about your work. By paying you are also, effectively making a contract with your talent and they are far less likely to bail on you or mess around. The actors who want paying are also probably the ones you want to work with.
Direct some theatre: I was so thankful that I was able to direct at Stockton Arc Scratch Night. It was a different environment to direct in, it made me think on my feet and definitely gave me more experience. Just getting as much experience prior to your film really helps you to. Being around performers and seeing how they work sinks into your subconscious and gives you knowledge you will use later
Read "Directing Actors" by Judith Weston: Just read the whole book. Then read it again. She really has some great ideas and techniques for rehearsals and working practically with your actors.
Make sure you know all the cast (if possible): Having 'rehearsals' over tea and cake and general chit chat are really helpful because everyone knows each other before they have to perform together and you become more comfortable with them. It gets rid of a whole lot of awkwardness. Of course, some of your actors who are only coming in for the day of the shoot from a long distance are difficult to meet up with and that's just life. In those cases just be super friendly as soon as you meet them and put them at their ease. Try and keep it professional though. The more you joke around the less authority you'll have. Find a balance.