How might teachers and students deepen dialogic space in online discussions centered on race? This paper explores challenges of creating shared spaces of collective inquiry online across audio/visual/written modes. We explore why participants switch modes--e.g. from oral/visual participation to written chat--while participating in a synchronous video call. We use examples from an online teacher-preparation course at a Southern US university to demonstrate how primarily White prospective/practicing teachers mode-switched during dialogue about Black language and linguistic justice. We identify common types of mode-switching whereby participants resist, revise, and renegotiate dialogic space in online coursework. Across examples, dialogic space emerged or deepened when writing "in the background"--before and/or during class--was foregrounded, bringing prior assumptions and present perspectives into creative tension with emerging understandings. Teachers might consider how relationships among modes like writing and talk, across activities and platforms, can support or inhibit dialogue in face-to-face or online spaces. (150)