Railpass use in France

France is one of the least pass-friendly countries for train travel, especially the TGVs that require seat reservations and have limited quotas for pass holders. So check your calculations carefully.

However, as always you should buy your tickets and seat reservations from the company that runs the train, SNCF in this case I presume. Avoid RailEurope and Trainline. For the TGVs, buy them in advance before they run out. Local and regional trains seldom require seat reservations.

BJ,

Keep in mind that reservations will cost you €10- €20/pp for TGV & Intercity trains, and.... there are a limited number of seats for pass holders.
Your best bet is to buy point to point discounted tickets in advance for your long journeys that require a TGV or Intercity train.
Short distance TER ( (regional) trains are inexpensive & there’s no need to book in advance.

Please read this invaluable website: Seat61.com/France-trains

this seems to be supported by adding up the point to point costs

Exactly how did you make this comparison? I have never found it cheaper to purchase a rail pass for rail travel within France, at least not in decades. Did you make comparisons with the assumption of purchasing a travel card such as one of the Cartes Advantage or Carte Liberté which offer additional discounts?

In the event one boards a French train and the conductor rejects your rail
pass, can they just take a credit card and sell you a ticket on the
spot? Is there a fine?

You should resolve this BEFORE boarding by making inquires at the station. You can purchase a ticket from an inspector but you need to search him out immediately upon boarding. If you do not, you most certainly will be fined on the spot. Even if you purchase tickets from an inspector upon boarding the train, you will likely pay a ticket premium.

Rail passes in France don't give you the flexibility you'd like because of mandatory reservations on 90% of routes out of Paris.
It is much better to buy tickets in advance for the day trips that interest you.
Most tickets are refundable with a nominal (typically 5€) fee, so you can book tickets for two dates, and choose one. You need more cash to do this (since you pay for both tickets) but it lets you benefit from advance-purchase fares (90-120 days early) while keeping some flexibility. I do this very often when planning weekends away.

For regional trains (TER), your pass is all you need.

For conventional long-distance domestic trains (Intercités), some require reservations and some don't. These passholder reservations are not capacity-controlled, but the train itself may sell out if you happen to be traveling at a busy time or other trains have been canceled. To see which routes require reservations, search for "destinations intercités". The page on SNCF's "Oui" site has a link to the "carte du réseau" (network map).

For high-speed trains (TGV = "Train à Grande Vitesse"), you do need a passholder reservation, and these may be capacity-controlled.

If you want to reserve in advance, on the Web, follow the intructions for passholder reservations for France on the Seat61 Web site. (You'll end up with Interail reservations, but they are identical to Eurail ones. If you are worried, an agent in any SNCF boutique in any station can later switch the tarif from Interail to Eurail.)

If you don't need to reserve in advance, SNCF "grandes lignes" (as opposed to regional) machines can sell passholder reservations. Tap "autres réductions" to find Eurail.

Note: In France, you will sometimes find a mix of TER, Intercités, and TGV trains serving the same itinerary with no speed advantage for the TGV because it's running on a conventional line, "ligne classique", on its way to a high-speed line, "ligne à grande vitesse".

A pass is quite handy when you have a long stay in a base city and don't know exactly when you'll take your day trips. You can check the weather, see how energetic you are on a given day, decide to leave early or to stay an extra hour, etc., which you cannot do with deep-discount advance-purchase point-to-point tickets. Also, passholder reservations within France are exchangeable without penalty, subject to seat availability, and you can exchange them using the machine.

Passes get a bad rap here but I maintain that when you're on vacation, flexibility and simplicity have value.

Regarding conductors, there is always a risk that you will have to pay a fine and the cost of a full-fare ticket if you lack a valid pass (note all of the conditions in the brochure: passport, pass activation, validity period, travel diary, and travel day if a flexible pass) and a reservation (where required).

On the other hand, if you are ready with your pass opened, travel diary filled out, and reservation displayed, French conductors will be impressed. If you want to travel like a pro, you can load your reservations -- however purchased, even if on paper -- into the "Assistant SNCF" app and display them there, or in Apple Wallet. If importing a reservation doesn't work, switch to "en gare" (bought at a station / NOT an e-ticket), enter the reservation number and train number, and voilà! In many cases, a French train conductor who has scanned a QR code on your phone won't even ask for the underlying pass.