It's safety -- the single most important thing you need to think about as you head out to your set to shoot your film. And even if you're a pretty sensible filmmaker with no plans to put your cast and crew in harm's way, there is still a lot of precautions you might be overlooking. Luckily, in this helpful video RocketJump Film School breaks down the basics of on-set safety, from who is responsible for it to typical safety measures one must take to ensure their team isn't being put at risk.
Making movies is tons of fun, but don't ever forget how serious working on set truly is. Even if you're not trying to pull off any explosions, car jumps, or stunts like that, you still need to be aware of the potential risks you're taking. (Not trying to be a downer here, but let's get real.) The Sarah Jones tragedy has taught us all that no shot is ever worth putting your cast or crew in any kind of danger.
So, if you have the budget to hire an Assistant Director, do it. They can put in place and manage the safety measures that your production needs, so you, as the director, can focus on getting the shots you need. This is one thing that you don't want to divide your attention on.
Unfortunately accidents are bound to happen on set, because nothing ever goes as planned. So, it's your job as a director/AD to make sure that you're making things as safe as humanly possible and not putting your cast, crew, and others at risk for injury or worse.
Source: RocketJump Film School