Pitch a good to great premise with lots of potential for all kinds of storylines, but waste it by letting it play out in the most pedestrian way you can think of. Standard situations galore!
The showdown has to be a race against time, no matter if this is at odds with the tone of what happened before or doesn’t feel organic at all.
Use (pop) songs that tell the audience how to feel, in the best case directly with their lyrics.
There has to be a tedious family-related subplot. Probably a neglected kid? An estranged dad? They have no other function than to serve the character arc of the leads. Best served with a reconciliation so unearned it makes the discerning audience furious.
Talking of kids, there has to be an annoying one. Or a petulant teenager. At one point though, the kid/teenager unexpectedly says something really insightful that causes a change of heart in the main character. Has to have a face that you want to smack really hard.
If possible, include a control room that is connected via radio/occasional video with the hero into the script. Fill the room with people representing all genders, races, ages and body types. They have to fully empathize with the hero, face palm when the hero fails or cheer and hug when he/she succeeds. Again, this helps the audience to know how to feel. Must include a scene where they think they lost contact only to suddenly hear the voice of the hero again, followed by relentless cheering.
One of the main characters has to be revealed as traitor shortly before the finale. Works best if none of his/her preceding actions make any sense in retrospect, like “staying in character” while completely alone, etc. The audience will groan!
Use the “reluctant hero trope”- but not lightly, lay it on thick! The hero has to have a major change of heart for at least 4 times over the course of the movie. In the end, shortly before the final battle (or match, if it is a sports movie) the hero has to disappear suddenly. But the wise mentor knows that he/she is hiding at her/his “secret favourite place” (rock, tree, rooftop) to contemplate and finds him/her there. This leads to a tedious confession by the hero about how he/she is afraid to fail and drag everybody down, followed by an equally tedious parable told by the mentor that makes the hero reconsider his/her decision. It’s not over yet though, first the team has to reject the returning hero, particularly the hardass of the group (Michelle Rodriguez/Adam Baldwin are preferred casting choices) has a few choice words, only to be put in the place by the mentor who reminds him/her of his his/her own past wrongdoings and mistakes.
To hammer the point home here: the keyword is “TEDIOUS”.
This segues into the next commandment, namely think up a pointless conflict between the leads that crops up when the plot stalls and you cannot think of a better device to inject some tension into the story. Possible variations are a) a misunderstanding based on a contrived communication error (works great in rom-coms) or b) an even more contrived decisional conflict that seems to reflect the differing ideologies/world views of the disputants, although they are actually rather similar on closer inspection. The utmost frustration potential is unlocked when the reasonably intelligent audience members can spontaneously think up 5 different compromises, while the characters cannot.
Prolonged. Farmhouse. Sequence.
More mediocrity here!