Honestly, just answering the problem that they have to solve in a way that their other options don’t. If you are starting out, own it and do your best to keep the client within reason.
If you are middle of the road pricing but its a project you can tackle, own that too. Don’t be afraid of letting them professionally take a walk if their pocketbook is quiet but their needs are many. There are design students and other lower prices platforms that will humble their ways and their time.
If you’ve done this before, your conversation will be just that with your potential client – a conversation. You don’t need to SELL to them, and they have an idea of what you can do is able to solve the problem you have.
You aren’t getting paid for how long or how quickly you come up with your solution – you are getting paid to solve their problem that they have at this time, and a few you’ve just helped them avoid by taking you on to navigate those waters.
Just be mindful of the start up time you are investing in the client, and understand that although its part of “getting to know the client” process… you aren’t getting paid for it.
Just be honest about your process, keep everything within reason in a statement of work or working agreement, for both parties, be realistic about your timetables and be sure to absolutely hit every due date necessary for them.
No surprises for the client equals a most likely client. The only thing they really want you to do is to underpromise and overdeliver. Just make it happen the best that you can, safely, professionally, and profitably.