It depends GREATLY on the configuration of your network. To simplify this, I'll assume you're running on a flat network.
To have Snort inspect all LAN>LAN traffic inline, you would need to ensure that there are no L2 devices interconnected below the Snort device,which I'm not sure is practical.
In a normal network configuration, all inter-LAN traffic would take place at layer 2, and would never reach the Snort box, as it would never leave the layer 2 switching infrastructure.
I don't know that there is a practical way to have Snort act as an IPS on entirely LAN-based traffic unless it's in-line at layer 3 between two different LAN networks.
Probably the best solution if it is a flat network you would like to monitor would be to mirror a port on an L2 switch and send all of the traffic to Snort there, acting as an IDS. Note that it must act as an IDS in this scenario because while it can SEE the traffic because it is being mirrored to it, it cannot act on this because it is not responsible for forwarding the traffic as it would be in an L3 inline configuration.