The uniqueness of a wedding speech
Wedding speeches are vastly different from other public speaking engagements.
A wedding speech is unlike pretty much any other speech you will ever be asked to give. Everyone in your audience will have their expectations, but no-one is paying to see you speak, and you will be doing it out of goodwill and, hopefully, love and respect for the couple getting married. Most people in the audience will have heard a wedding speech before, and will be hoping for something different, something fun, something interesting, but something that is also bespoke and relevant to the happy couple.
No-one expects a professional level of presentation, and your audience will be relaxed (possibly helped by some alcohol inside them!) about you fluffing the odd joke or perhaps falling back on a tried and tested gag, but will also be expecting something inoffensive, personal and amusing. You need to tread a fine line between being entertaining, perhaps poking some gentle fun at the groom and his life so far, whilst being ultimately complimentary, and avoiding giving any serious offence to the bride, or indeed anyone else present. Don't forget, the audience is likely to contain your friends and family too; this isn't a business speech where the audience are strangers who you will never see again. These people are likely to be in your life for a long time, and whilst they are rooting for you to be entertaining and funny, you need to put the work in beforehand in order to satisfy their expectations.
Being asked to give a wedding speech is never less than a compliment, and an indication that the couple are putting their trust in you to deliver something apt. In return, the least you can do is to spend some time preparing something that truly reflects the lives and personalities of the bride and groom, and your good wishes for their future.