Make your Offer Match your Intention.
This is first because, to me, this is where the disconnect occurs and why your opt-in will not convert—this seems overlooked by many experts but should be part of the checklist to make sure it’s clear for the user.
So, ask for someone’s email because you are going to send them something… Period.
YOU DON’T NEED someone’s email address to play a video.
YOU DON’T NEED someone’s email address so they can download a document.
People can access things for free, without giving up their email all over the net, so if the user experience is such that the person has to stop and think, “Why is this person asking for my email?”, they will bounce. This also clearly shows your intention, and that what you are really after, in a backhanded way, is their personal data so you can sell them something else. Not cool!
If it’s clear to the user that the only way they can get your content is via email, then it all makes sense that you need it for this one reason… Oh!, as well as to re-market to them later
If you have a 5 or 10-part series of something, or need to send emails to the user more than one time, this can also be a much clearer way to gain their email and for them to feel comfortable giving it. Makes sense?