Please note that from now on when I say "director", I also mean the producer since in micro-budget films they are often one and the same.
It is rare for me to be called in to work on a film during the very early stages of pre-production. There are some films I've done that I wish I could have had some sit-down time with the director early in the game in order to plan out how to shoot what he'd like to have seen rather than being rushed to toss something together with very little prep time. (To be fair, many folks that I've worked with in the past now give me much more time to prepare for their projects.)
Good practical FX do not necessarily require a lot of money, but they often need some time to figure out and develop. As soon as I get a "kill list" from a director who doesn't have a very clear idea of how he wants to see the scene go down, I immediately start thinking about different ways to execute the effects. If given enough time, it's not uncommon for me to come up with ideas to improve on the effects and discuss them with a director.
For example, when I first joined the crew of Dangerous People, the director had a pretty good idea of what he wanted to see. We did a few very simple-but-effective effects, although I would have liked to have gone a bit farther with them. It was really a blessing in disguise when the production came to a screeching halt on account of the unprofessional behavior of a key crew person and a main actor. Instead of just scrapping the film then and there, the director decided to completely re-shoot the film. This ultimately led to a discussion about how to improve some of the effects and taking the time to create them, which resulted in some truly interesting visuals for his movie.
Communication between the FX artist and the director is crucial when planning out a scene. I've learned that many directors really have no idea how long applying a prosthetic piece to an actor can take, and many effects gags require shooting in multiple-stages and camera angles (and later on specific editing instructions) in order to sell the effect to the audience.
My advice to anyone working special FX on a guerrilla film : If a director tells you that you'll have at least four hours of prep time, expect to actually get about two and plan for that. Anything you can prepare before the shoot to save time is a real boon for you... mix your blood, pre-paint your prosthetic pieces, find out about any allergies an actor might have, etc...
For a great way to experience high-pressure and zero prep time situations, I recommend joining a team for a 48-hour film challenge. You will literally have to make things up as you go along using whatever you happen to have on hand. It's like a blind date for film crews.
As an FX artist working on micro-films, you have to be both imaginative and thrifty... and also be prepared to switch gears on very short notice. When I plot out effects I always try to have at least two plans on how to execute them in place, knowing that my original plan might end up not being feasible on account of the location, actors, time allowance, weather, etc... For example, I love using specially-built devices for doing gunshots. However, I always bring along a hand-pump just in case there is no appropriate electrical source available on the set to plug in an air compressor.
Sometimes a director will ask for an effect that is pretty much impossible to pull off with the time or money allowed. This is why I can't stress enough how important it is for directors and FX artists to sit down and discuss the effects as early on in production as possible.
For example, a very realistic decapitation suitable for close-up shots will require a head cast of the actor as well as a lot of time spent on making the detailed head prop with materials that can get pretty expensive. There are other alternatives to get a really convincing severed head, but they often require building specialized props or literally cutting holes in the sets. None of these can be done effectively in twenty minutes on a $30 budget *.
When talking to a director, be realistic about your abilities. Don't tell him that you can turn someone into a zombie in less than five minutes unless you are planning to just throw pre-made rubber masks on the actors. Directors are depending on you to be able to come through on your word, so don't just tell them whatever you think they want to hear just to land the gig. Remember, most directors have never worked with materials like scar putty and liquid latex. Explaining to a director exactly why a good zombie makeup will take longer than five minutes usually leaves a very favorable impression and they are often willing to schedule accordingly in order to improve the production value of their film.
* Yes, someone actually asked me for that once.