How does Sam Know?

[Kent2010]

I don't think in this instance the screenwriters altered the meaning of the text all that much. Obviously in the movies when all the companions pledge there support the dialogue is completely made up. And in the books The Council ends with Frodo saying he'll take the Ring and Sam jumping in. Later Elrond informs Frodo of his other companions and Merry and Pippin are the last two. All the screenwriters did was condense and have the entire Fellowship form during The Council of Elrond. I think it was necessary condensement to keep the movie going and the Fellowship out of Rivendell. For this to happen, yes dialogue had to be invented and it does look like some companions pledge an oath to Frodo.

But, I find Elrond's choice of words, regarding any oath on the Fellowship important:

"You may tarry, or come back, or turn aside into other paths as chance allows. The further you go, the less easy will it be to withdraw; yet no oath or bond is laid on you to go further than you will..."
-The Ring Goes South

There is no oath on how long, or far, anyone on the Fellowship needs to stay with Frodo. Elrond does make it clear that the members are free, they can leave Frodo at anytime. However, right before the quote above Elrond says:

"The Ring-bearer is setting out on the Quest of Mount Doom. On him alone is any charge laid: neither to cast away the Ring, nor to deliver it to any servant of the Enemy nor indeed to let any handle it, save members of the Company and the Council, and only then in gravest need. The others go with him as free companions, to help him on his way."
-The Ring Goes South

Only in the gravest need is anyone of the Company allowed to handle the Ring, and this is of course at Frodo's discretion. There may not have been any sworn oaths, or 'swords,' as shown in the movies but there were conditions to being a member of the Fellowship. That is only Frodo can handle the ring, unless he lets another member if need calls for it, and the company is to help Frodo on his way. Boromir broke the second. Now whether he broke a sworn oath or not, I don't know. But I will ask what did Boromir need forgiveness from if he was completely free and not bound by any oath while in the Fellowship?

There were far worse diversions from the script than the scenes being discussed. Yes the dialogue is completely made up, but the condensing of forming the Fellowship during The Council can be argued as being necessary. While no oath was placed on how long someone decided to stay in the Fellowship, that doesn't mean there were no bonds at all. There may not have been an explicit oath, but Frodo's 'charge' was to bear the Ring to it's destruction and I think it can be implied the Fellowship's duty was to help him on his quest. Boromir broke these when he tried to take the Ring by force.