Always be thinking your title and thumbnail are one and the same. They're not separated.
They are working together to stack as many reasons as possible to get the click.
Here's a good example:
my new video titled: "make videos for a specific person"
thumbnail text: 'not the algorithm" (adds more context to the title)
Circle and dot (adds further contexts it's actually about one person)
red arrow leads attention to the graphic enhancing the message in the title
picture of me: indicates is a talking head video or I'm the subject in the video.
Bad example:
- repeating words in your thumbnail that are in your title (wasting viewers time by reading same info twice)
- not adding more context in your thumbnail text
- not revealing key message or clue of the video
Further on from this is the introduction which is to reinforce the reason they've clicked on the video and give a quick win (value) as soon as possible.