You can take care of some details beforehand so everything is ready when the painters arrive for the paint job. If painters have to do the prep work of removing electrical outlet covers and light fixtures throughout the interior, they'll likely charge that extra time to you. Move large furniture to the center of the room, and cover everything you don't want paint-splattered with a drop cloth.
Additionally, some painters may reduce the cost of the job if you provide materials they would otherwise have to, such as paint covers, brushes, floor/furniture coverings, and paint trays and rollers.
You also might be able to save on your painting contractor's labor cost by taking on some do-it-yourself work, such as doing a touch up on corners. Talk to your painter to see if there's any way you can pay less by making their job easier or faster.
And always start with an estimate. Getting estimates from multiple pros will equip you with the knowledge to confidently hire a pro and tackle an interior painting project.