Wikipedia:Read before commenting

Discussions on Wikipedia are a collaborative process, and it works better when editors build off of each other's ideas rather than just jumping in with their initial thoughts. When joining a discussion, it is always best practice to read through the prior comments and familiarize yourself with the context before contributing your own thoughts.

Why you should read first

 * The issue might be different than it seems. The initial editor might not have presented the issue neutrally, or circumstances may have changed since the discussion was opened. Reading it will allow you to notice this and save you the embarrassment of being called out by others and possibly whacked with a wet trout.
 * Someone else might change your mind. Being open to persuasion is not a sign of weakness, but rather of intelligence. Consider the example of John Maynard Keynes, who when accused of flip-flopping, famously replied "When my information changes, I alter my conclusions. What do you do, sir?" It's okay if you disagree with others, but you should understand why they have the perspective they have.
 * Someone else might have already made your point. If this is the case, you can help prevent the conversation from sprawling by just writing [your stance] per [editor] or, for less structured discussions, using Agree. This also saves you the effort of writing out your point and earns you goodwill with the editor you are seconding.

If you don't feel like reading a long discussion, that's alright. But in that case, it's often better not to comment at all than to comment while uninformed. Trust that those who do decide to engage will work things out for the best, and feel good knowing that you've done your small part to help keep the discussion from becoming even more unwieldy.

Exceptions
This essay should not be interpreted to mean that an editor who bludgeons a conversation with long rambling comments can force you to read them before you're able to participate. Everyone should aspire to write concisely, and those who fail to do so forfeit the right to others' attention.

As a matter of practicality, there are also some discussions that are just too long to read through. For these, skimming can still help, and acknowledging that you haven't fully caught yourself up can help others help you if there's anything you missed. If you do go the extra mile and read the whole thing, consider measures that could help others joining after you, such as putting together a summary, closing resolved portions, or collapsing off-topic tangents. And feel extra-good about yourself.

Civility
Please be careful about linking to this essay in discussions. Don't assume that an editor has not read your comments just because they do not agree with them. They might just be stubborn, or (gasp) you might not be as persuasive as you think.