Can I appoint someone to be my Executor / Trustee / Guardian without informing them?

Technically speaking, your will would still be valid even if you DO NOT inform your would-be Executor / Trustee / Guardian.

BUT...

The said appointed roles (Executor / Trustee / Guardian) INCLUDING your beneficiary, have the right to refuse these roles or might be unable to act at that point in time (e.g. if they are no longer around.)

While the odds of your beneficiary refusing to be your beneficiary as they do not want to accept the gift is relatively slim, roles which requires work and fiduciary duties such as the Executor / Trustee / Guardian are another matter entirely. Letting them know early would help pre-empt them, and at the same time allow you to know beforehand if they don't want to so you can look for someone else. (You can see how it's going to be too late if it happens after your death)

That being said, it is generally recommended to appoint substitutes for these roles as well.

Here at getArrange.com, we understand the importance of having substitutes which is why we DO NOT charge people by "per clause"; you shouldn't have to pay more just because you want to add more substitutes as contingency.