Your bot is a representation of you— that is, of your brand— and the way your bot communicates will become part of your brand voice. Now, you might just be two people in a room who have put together an amazingly useful bot and want to get it into the world (and into Slack), and you're thinking, "But we don't have a brand voice."
Well, you do now. Even if the only place it's used right now is this one bot.
So you need to be thoughtful about defining what you brand voice is. Unless you're bringing in a seasoned professional writer to give the thing a real personality, the best thing to do is think of this voice as an extension of your own voice.
Clear, concise and human. If you stick to those things, you can't go far wrong. Whatever you're trying to say, think about how you would explain it first. Don't leap into "what words would my bot choose." You'll probably end up picking bizarre ones.
You might find it helpful to jot down some attributes or characteristics of your brand voice (e.g. friendly, authoritative), once you've thought about it. MailChimp's Voice and Tone guide is a widely-admired example.
Your bot's primary purpose is not to sound clever or to entertain your users, it's to help them accomplish a task— even if that task is inherently entertaining, like finding just the right cat GIF). However useful the service that your app is providing, it's all going to be for nothing if you annoy people so much they'd rather go through whatever laborious process they were using before than have to deal with your overly chipper bot one more time.
You want to be able to differentiate yourself from the crowd, for sure, but try adhering to these guidelines:
A little goes a long way. We cannot say this enough.
Don't add a joke or aside just to add one—almost every word your bot says should facilitate an interaction (courteous parts of speech, such as greetings, are also useful).
The second example still has plenty of distinctive personality, but gets straight to the point, and doesn't risk users tuning out/not wanting to wade through unimportant content.
Try to write copy for your interactions that someone who doesn't speak your language fluently could easily understand. That means:
The first example includes a reference to an obscure film that's likely to confuse many more users than it delights. The emoji combination is also potentially confusing, and may stall some users as they try to decipher it ("Fire... meat? Firemeat?").
The message button copy is similarly difficult to understand, and, in the worst case, could prevent users from selecting a response at all. Try to use standard combinations on buttons (Attend/Decline, Confirm/Cancel) to help your users have a smooth, simple experience with your app.
Read over your copy and ask yourself, "Is there anywhere a user may pause in confusion?" If so, rewrite.
Slack users are people of all ages, races, genders, and ability levels. They may have poor internet connections, use Slack only on mobile, or be forced to use Slack by their boss. We want them all to have a great experience on Slack.
That means bot makers should:
Writing for a broad audience takes a bit of practice if you're new to it, and it is usually easier if you have a team of people from diverse backgrounds working on your bot from the start.
We spend a good amount of time at Slack writing, rewriting, agonizing over, and then rewriting just once more to get every sentence as good as we can make it. If you've worked with a professional writer before, you know that no one, not even the people who make a living at this, gets it right the first time.
Here are some techniques we find helpful in revising:
After a few rounds of revision, your bot should be ready to help your users accomplish tasks in Slack, while avoiding confusion, frustration, and consternation. Congratulations!
Words and copy used in your interactions should be easily understood even by someone who doesn't speak the same language fluently.