A will wholly in the testator's handwriting is valid without further requirements, provided that the fact that a will is wholly in the testator's handwriting and signed by the testator is proved by at least two disinterested witnesses.
A willnot wholly in the testator's handwriting is not valid unless the signature of the testator is made, or the will is acknowledged by the testator, in the presence of at least two competent witnesses who are present at the same time and who subscribe the will in the presence of the testator. No form of attestation of the witnesses shall be necessary.
In England and some states, the line of sight test is used for presence. It requires the testator to have been able to see the witnesses sign the will if he looked. You have to be in line of sight. You can't be in a nearby room or something.
UPC § 2-502(a)(3) does not require that the witnesses sign in the testator's presence at all. It does require them to see the testator's signing or acknowledgment of the will. If the testator has another sign on his behalf, the UPC requires conscious presence.
Attestation clauses recite that the will was executed in accordance with the applicable Wills Act. No state normally requires an attestation clause, but they give a rebuttable presumption of due execution, so you should always have one.
A slim majority of states have purging statutes, which purge benefits that witnesses to wills receive therefrom. Most of them only purge the benefits in excess of what the witness would have received in intestacy however.
If there are sufficient witnesses to a will without the interested witness, then that witness was supernumerary and may take his full devise.