A modal appears in front of your product and demands attention. That makes it the most effective format here and the most expensive one, because a customer who didn't want it has to deal with it before continuing.
A modal takes a heading, text, and header media — an image or a video. For video you control whether it plays by itself, whether it loops, and whether the controls are shown.
Autoplay without controls is a combination worth avoiding. Someone who can't pause a video they didn't ask for closes the modal instead, which costs you the message and some goodwill.
It can carry an action too — a button opening a URL, in a new tab if you like, or triggering something inside your product by name. You can also have the messenger close when the action runs, so people land where the action takes them.
There's a setting for whether the modal can be dismissed. Leave it on for anything short of a blocking requirement.
Very little. The honest list is: something the customer must act on before continuing, and something so significant they'd be annoyed to have missed it.
A new feature is usually news or a tooltip. A maintenance window is a banner. A question is a survey. Reach for a modal when the alternative is a customer being surprised later.
Set the frequency to once. A modal that reappears is the single most reliable way to make people dislike a product, and “once” is almost always what you meant.
Avoid showing it the moment someone arrives. A modal in front of a page that hasn't finished loading gets dismissed without being read — waiting until they've settled costs nothing and is the difference between being read and being closed.
